In U.S. Pat. Nos. 5,134,107 and 5,403,807 commonly assigned herewith, sol-gel processed oxide materials are disclosed which are useful for catalysis in automotive exhaust gas systems. The oxide materials of the '807 patent contain alkaline earth metals and optionally lanthanides in an alumina matrix. It was disclosed that the sol-gel processing allows for the low temperature preparation of aluminum oxide materials of high purity and controlled microstructure with excellent high temperature stability. These materials can be in such forms as gels, aerogels and xerogels.
We have now found that zirconia based materials made by sol-gel techniques using heterometallic alkoxides to incorporate alkali metal and/or alkaline earth metal with the zirconium are excellent materials for use in automotive catalysis for example as catalyst supports or for NOx absorption.
Significant research has been undertaken in the treatment of exhaust gas generated by lean-burn engines which includes hydrocarbons, carbon monoxide and nitrogen oxides. In such lean-burn gasoline and diesel engines, the A/F ratio is higher than stoichiometry (oxidizing) which improves fuel economy. Since the exhaust is oxidizing, nitrogen oxides which must be reduced pose the most difficulty for conversion. One current approach to treating oxidizing exhaust gasses involves materials which act as NOx absorbents, i.e., materials which are able to absorb nitrogen oxides from the exhaust gas during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. For example, when the A/F ratio is made rich or stoichiometric. Conventional NOx absorbents are alkaline earth metals like barium with a precious metal catalyst like platinum carried on alumina. The widely held mechanism for this absorption phenomena is that during lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the trapping material, e.g., the barium. In the regeneration mode, under a stoichiometric or rich environment, the nitrate decomposes and the NOx released is reduced catalytically over the platinum with reducing species like HC or CO in the exhaust gas.
Such conventional absorbent materials have a serious deficiency in that the barium reacts with sulfur oxides generally present in exhaust gas to form barium sulfate. This inactivates the barium for NOx absorption. It is suggested that to decompose the barium sulfate it should be subjected to elevated temperatures of at least 600.degree. C. or more in reducing gas conditions. One negative aspect of this regeneration process is that it leads to detrimental changes in the NOx absorbent such as reduced surface area and crystallization of the aluminate phases thereby reducing the efficiency of the NOx absorbent. Alkali metals like potassium have also been suggested as NOx absorbents, however, they are even more easily deactivated by sulfur than alkaline earth metals like barium. Repeated regeneration of the absorbent by heating, as discussed above, contributes to a loss of surface area in the alumina support material and contributes toward further sintering in the platinum precious metal responsible for the conversion of NOx to NO.sub.2. Precious metal sintering results in a decrease in the active sites that convert NOx to NO.sub.2, and hence a decrease in the total amount of NOx trapped on the available absorbent.
In commonly owned application entitled "Use of Sol-Gel Processed Alumina-Based Metal Oxides for Absorbing Nitrogen Oxides in Oxidizing Exhaust Gas" (U.S. Ser. No. 09/184,146), we disclosed that sol-gel processed oxides particularly those made from heterometallic alkoxides like those of the '807 patent have excellent NOx trapping efficiency. We believe that this improved trapping performance of the sol-gel processed oxide material, as compared to conventional alumina impregnated with barium, is due to the atomic dispersion of barium in the alumina matrix due to the sol-gel process. It is also believed to increase the sulfur poisoning resistance in these alkoxide materials.
The present invention materials have superior NOx trapping ability when compared to similar formulations made instead by impregnating the metal such as alkali metal onto conventional zirconia. This NOx trapping ability operates under diesel and gasoline lean burn-NOx conditions. These and other advantages of the present invention materials will be discussed in detail below.
In U.S. application Ser. No. 09/134,992 filed Aug. 17, 1998 and entitled "NOx Trap Catalyst For Lean Burn Engines" commonly assigned with the present invention, a sol-gel oxide material is disclosed useful for NOx absorption. It comprises oxides of aluminum, magnesium and zirconium.